Sealing a pond to prevent water loss is a common challenge, and sodium bentonite clay offers a practical, natural solution. Bentonite is a type of clay composed primarily of montmorillonite that exhibits a remarkable ability to swell significantly when it contacts water, expanding up to 15 to 30 times its dry size. This expansion creates a dense, gel-like, low-permeability barrier within the soil. Utilizing this natural property, bentonite provides an environmentally friendly and long-lasting method for creating a waterproof seal in earthen ponds.
Assessing the Site and Preparing the Pond
The success of a bentonite seal depends heavily on properly evaluating the pond’s soil and preparing the site. Sodium bentonite is particularly effective in sealing porous soils, such as those with high content of sand, gravel, or silty-sand. Soils with existing high clay content or excessive organic matter may require less bentonite. Highly alkaline water may interfere with the clay’s swelling action, so water chemistry should also be considered.
Sealing a pond generally requires draining it completely to expose the entire surface area needing treatment. Once dry, the area must be cleared of all debris, including large rocks, roots, vegetation, and organic material that could compromise the clay layer. The soil moisture content must be optimal for compaction; if too dry, light sprinkling may be necessary, but if too wet, the project should be postponed. The goal is a clean, level sub-surface ready for the bentonite application.
Determining the Correct Amount of Clay
Calculating the necessary amount of bentonite relies on the total surface area to be covered and the porosity of the native soil. The first step involves accurately measuring the surface area of the pond bottom and sides that will be sealed. This measurement provides the square footage required for the application rate calculation.
The application rate, measured in pounds per square foot (lbs/sq ft), varies substantially based on the soil composition. For soils with high clay content, the required rate is lower, often between 1.0 to 1.5 lbs/sq ft. Highly porous materials like clean sand or gravel demand a much higher rate, ranging from 4.0 up to 6.0 lbs/sq ft to ensure a complete fill. For very deep ponds, it is recommended to increase the application rate by an additional pound per square foot for every eight feet of depth.
Bentonite is commonly sold in powdered or granular forms, and the choice depends on the application method. Powdered bentonite is preferred for methods where it is mixed directly with the soil due to its finer particle size. Granular bentonite is better suited for sprinkling over standing water because the heavier particles sink more effectively. Purchasing the correct quantity, which can range from 18 to 36 tons for a quarter-acre pond, is essential to avoid running out during the sealing process.
Methods for Applying Bentonite
The most effective and preferred method for new construction or fully drained ponds is the Blanket Method. This technique ensures the bentonite is confined and thoroughly integrated into the soil structure. It involves excavating the top 4 to 6 inches of soil from the pond bottom and banks, setting this material aside for later use.
The bentonite is then spread evenly over the exposed sub-soil at the calculated rate, aiming for a consistent layer across the entire area. For large areas, agricultural spreaders can be used, while smaller areas can be managed by hand, taking care to wear a dust mask and gloves. Any deep holes or crevices should first be pre-filled with a mixture of one part bentonite to five parts soil to ensure a smooth base.
The soil that was initially removed is then carefully replaced on top of the bentonite layer. This protective cover of soil, typically 4 to 6 inches thick, is important for confining the clay and preventing it from being disturbed or washed away. The final step in this method involves mechanical compaction to activate the seal.
A less effective but sometimes necessary technique is the Sprinkle or Broadcast Method, used when draining the pond is not feasible. This method is reserved for spot treatments of isolated leaks or when standing water is present. Granular bentonite is broadcast over the water’s surface, sinking to the bottom where it is drawn into porous leak areas. Uniform application is difficult, and it is only about half as effective as the blanket method because compaction is not possible.
Final Compaction and Refilling
Following the blanket method application, mechanical compaction is necessary to compress the soil-bentonite layer and activate the clay. Compaction forces the bentonite particles into the soil’s pore spaces, creating a dense, impermeable barrier. This is typically achieved using heavy equipment like a sheepsfoot roller, or by driving over the area repeatedly with other heavy machinery, ensuring four to six passes.
Proper soil moisture is a determining factor for effective compaction, as the soil needs to be at its optimum moisture content to achieve the required density. After compaction, the bentonite blanket needs time to cure and begin hydrating before being covered with water. This curing time allows the clay to settle and the soil to bond; it should be protected from drying out by a layer of mulch if a significant delay is expected before refilling.
When the curing period is complete, typically 24 hours to a few days, the pond can be refilled. It is important to refill the pond slowly and gently to prevent erosion or disturbance of the bentonite layer. A slow influx of water minimizes shear forces on the seal, allowing the bentonite to fully swell and form a lasting, water-tight barrier.